This invention relates to an ultrasonic method and system that utilize both fundamental and harmonic echo ultrasound information to provide an improved ultrasound image.
In most cases, ultrasound imaging systems create images using a transmit ultrasound beam formed at one frequency and a received ultrasound beam formed at a similar frequency. On occasions, the received beam center frequency is modified to accommodate to some extent the effect of depth and frequency dependent attenuation.
Recently, systems have been described which form receive beams and images using harmonic frequency signals. In this case, the transmit beamformer forms a transmit beam in a passband centered on a fundamental frequency, and the receive beamformer processes received echo ultrasound information at a passband centered at a harmonic of the fundamental frequency. Such systems can operate with additional non-linear contrast agent as described in Uhlendorf U.S. Pat. No. 5,410,516, or alternately they may operate without additional non-linear contrast agent, relying on tissue generated harmonics, as disclosed in the article entitled "Finite Amplitude Distortion-Based In homogeneous Pulse Echo Ultrasonic Imaging" by Ted Christopher (IEEE Trans UFFC, Vol. 44, No. 1, pp. 125-139, January 1997). Harmonic imaging has been found to provide a superior image quality in terms of detail and contrast characteristics under certain scanning conditions. However, penetration into the tissue is severely degraded, at least in midfrequency transducer probes, due to the fact that harmonic signals are formed from only a fraction of the transmitted power, and higher frequency signals are more severely attenuated by body tissues.